Add warning to demux documentation

[shlok]

I apparently need to do this at one point for things to go smoothly. (However, if this makes no sense (based on the way demux is known to work), feel free to reject.)

[harendra-kumar]

This should not be the case unless there is some laziness somewhere. Can you test and confirm that this makes a difference?

[harendra-kumar]

I apparently need to do this at one point for things to go smoothly.

Can you recheck/reproduce this?

[shlok]

@harendra-kumar The issue occurs if the folds are executed on different threads. Here is an example:

{-# LANGUAGE ScopedTypeVariables #-}

module Main where

import Control.Concurrent
import Data.Function
import qualified Streamly.Data.Fold as F
import qualified Streamly.Data.Stream.Prelude as S
import qualified Streamly.Internal.Data.Fold.Concurrent as F

main :: IO ()
main = do
  let foldEven =
        F.parEval id $
          F.foldlM'
            (\() x -> putStrLn $ "Processing even: " ++ show x)
            (return ())
      foldOdd =
        F.parEval id $
          F.foldlM'
            (\() x -> do putStrLn $ "Processing odd: " ++ show x)
            (return ())

  (io, _) <-
    S.fromList [0 :: Int .. 20]
      & S.fold
        ( F.demuxIO
            (\i -> if even i then "even" else "odd")
            ( \i ->
                if even i
                  then return foldEven
                  else return foldOdd
            )
        )
  -- _ <- io -- Commented out for now.

  putStrLn "Doing additional things..."
  threadDelay 10
  putStrLn "Complete."

Example runs:

$ cabal build; cabal exec -- demux-test
Up to date
Doing additional things...
Processing even: 0
Processing odd: 1
Complete.
Processing even: 2

$ cabal build; cabal exec -- demux-test
Up to date
Processing odd: 1
Doing additional things...
Processing even: 0
Processing odd: 3
Processing even: 2
Complete.
Processing odd: 5

This is a problem if the “additional things” rely on the “processing” being fully finished.

After uncommenting that _ <- io line, things work as expected:

Processing even: 0
Processing even: 2
Processing even: 4
Processing even: 6
Processing even: 8
Processing even: 10
Processing odd: 1
Processing even: 12
Processing odd: 3
Processing even: 14
Processing odd: 5
Processing even: 16
Processing odd: 7
Processing even: 18
Processing odd: 9
Processing even: 20
Processing odd: 11
Processing odd: 13
Processing odd: 15
Processing odd: 17
Processing odd: 19
Doing additional things...
Complete.